linux-kernel - Re: spi transfer with zero length

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Message-Id: <200802161058.49473.david-b@pacbell.net>
Date:	Sat, 16 Feb 2008 10:58:48 -0800
From:	David Brownell <david-b@...bell.net>
To:	Atsushi Nemoto <anemo@....ocn.ne.jp>
Cc:	spi-devel-general@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org
Subject: Re: spi transfer with zero length

On Saturday 16 February 2008, Atsushi Nemoto wrote:
> Hi.  Is it legal to use zero for 'len' field of struct spi_transfer?
> I mean, len=0, tx_buf=rx_buf=NULL, delay_usecs!=0.

Yes that should work ... it's uncommon, but not illegal.  Some
controller drivers may even handle that right!

If the delay were zero and cs_change didn't indicate a need to
briefly deselect the chip, it might make sense to reject such
a NOP transfer.  But that's not the case you identify.

> Some SPI devices need slightly long delay before first CLK edge after
> CS assertion.

For future reference ... could you identify a few such devices,
and say what "long" is relative to the clock period?

Some folk have just slowed down the clock in such cases, but
that's rather sub-optimal.

> To achieve this, I think inserting using a zero length 
> transfer before real transfers.  But it seems some drivers do not
> handle this case properly.

Feel free to submit patches fixing those bugs.

> Is this driver's bug, or we need additional delay field in struct
> spi_device for such case?

I'd like to avoid new parameters to cover case that can already
be expressed in the programming interface.  Cases that can't be
expressed ... different issue.  I suspect any patches updating
timing parameters should use nanoseconds not microseconds, fwiw.

- Dave
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